Micromechanical devices are small structures typically fabricated on a semiconductor wafer using techniques such as optical lithography, metal sputtering, plasma oxide deposition, and plasma etching which have been developed for the fabrication of integrated circuits.
Digital micromirror devices (DMDs), sometimes referred to as deformable mirror devices, are a type of micromechanical device. Other types of micromechanical devices include accelerometers, pressure and flow sensors, gears and motors. While some micromechanical devices, such as pressure sensors, flow sensors, and DMDs have found commercial success, other types have not yet been commercially viable.
Digital micromirror devices are primarily used in optical display systems. In display systems, the DMD is a light modulator which uses digital image data to modulate a beam of light by selectively reflecting portions of the beam of light to a display screen. While analog modes of operation are possible, DMDs are typically operated in a digital bistable mode of operation and as such are the core of the first true digital full-color image projection systems.
Micromirrors have evolved rapidly over the past ten to fifteen years. Early devices used a deformable reflective membrane which, when electrostatically attracted to an underlying address electrode, dimpled toward the address electrode. Schlieren optics were used to illuminate the membrane and create an image from the light scattered by the dimpled portions of the membrane. Schlieren systems enabled the membrane devices to form images, but the images formed were very dim and had low contrast ratios, making them unsuitable for most image display applications.
Later micromirror devices used flaps or diving board-shaped cantilever beams of silicon or aluminum, coupled with dark-field optics to create images having improved contrast ratios. Flap and cantilever beam devices typically used a single metal layer to form the top reflective layer of the device. This single metal layer bent downward over the length of the flap or cantilever when attracted by the underlying address electrode, creating a curved surface. The curved surface scattered incident light--lowering the contrast ratio of images formed with flap or cantilever beam devices.
Torsion beam devices were developed to improve the image contrast ratio by concentrating the deformation on a relatively small portion of the DMD surface. Torsion beam devices use a thin metal layer to form a torsion beam, which is often referred to as a torsion hinge, and a thicker metal layer to form a rigid member. The thicker rigid member, which is sometimes referred to as a torsion beam or simply a beam, typically has a mirror-like surface. The rigid mirror remains flat while the torsion hinges deform, minimizing the amount of light scattered by the device and improving the contrast ratio of the device.
Recent micromirror configurations, called hidden-hinge designs, further improve the image contrast ratio by using an elevated mirror to block most of the light from reaching the device superstructure. The elevated mirror is connected by a support post to an underlying torsion beam or yoke. The yoke is attached to torsion hinges which in turn are connected to rigid support posts. Because the structures which support the mirror and allow it to rotate are underneath the mirror instead of around the perimeter of the mirror, virtually the entire surface of the device is used to fabricate the mirror. Since virtually all of the light striking a hidden-hinge micromirror device reaches an active mirror surface--and thus either used to form an image pixel or reflected away from the image to a light trap--the hidden-hinge device's contrast ratio is much higher than the contrast ratio of previous devices.
Micromirror devices are generally operated in one of two modes of operation. The first mode of operation is an analog mode, sometimes called beam steering, wherein the address electrode is charged to a voltage corresponding to the desired deflection of the mirror. Light striking the micromirror device is reflected by the mirror at an angle determined by the deflection of the mirror. Depending on the voltage applied to the address electrode, the cone of light reflected by an individual mirror is directed to fall outside the aperture of a projection lens, partially within the aperture, or completely within the aperture of the lens. The reflected light is focused by the lens onto an image plane, with each individual mirror corresponding to a location on the image plane. As the cone of reflected light is moved from completely within the aperture to completely outside the aperture, the image location corresponding to the mirror dims, creating continuous brightness levels.
The second mode of operation is a digital mode. When operated digitally, each micromirror is fully deflected in either of the two directions about the torsion hinge axis. Digital operation uses a relatively large address voltage to ensure the mirror is fully deflected. The address electrodes are driven using standard logic voltage levels and a bias voltage, typically a positive voltage, is applied to the mirror metal layer to control the voltage difference between the address electrodes and the mirrors. Use of a sufficiently large mirror bias voltage, a voltage above what is termed the threshold voltage of the device, ensures the mirror will fully deflect toward the address electrode--even in the absence of an address voltage. The use of a large mirror bias voltage enables the use of low address voltages since the address voltages need only slightly deflect the mirror prior to the application of the large mirror bias voltage.
To create an image using the micromirror device, the light source is positioned at an angle relative to the device normal equal to twice the angle of rotation so that mirrors rotated toward the light source reflect light in a direction normal to the surface of the micromirror device and into the aperture of a projection lens--creating a bright pixel on the image plane. Mirrors rotated away from the light source reflect light away from the projection lens--leaving the corresponding pixel dark. Intermediate brightness levels are created by pulse width modulation techniques in which the mirror rapidly is rotated on and off to vary the quantity of light reaching the image plane. The human eye integrates the light pulses and the brain perceives a flicker-free intermediate brightness level.
Full-color images are generated by using three micromirror devices to produce three single-color images, or by sequentially forming three single-color images using a single micromirror device illuminated by a beam of light passing trough three color filters mounted on a rotating color wheel.
While demand for micromirror-based display systems is created primarily as a result of the superior image quality the systems provide, some market segments are characterized by cost concerns more than image quality concerns. Micromirror devices are produced in bulk on semiconductor wafers and therefore take advantage of the same wafer processing economies of scale which characterize the semiconductor industry. Wafer processing places great emphasis on the wafer yield--the number of working devices produced by each wafer. Therefore, methods of increasing the wafer yield are needed.